Bob McDavitt's ideas for sailing weather around the South pacific

08 April 2018

Bob Blog 8 April




Compiled Sun 8 April 2018


Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.


One of my blog followers, who uses saildocs to receive this weathergram, informed me last week that he received a spam email using my email address Has anyone else received anything?


Fifty years ago, on Tuesday 10 April, a storm from the tropics met with a cold front just off Wellington and re-intensified. As a result, the Inter-island ferry WAHINE sunk and 51 lives were lost that day. Our house lost its chimney. The storm was terrifying and tragic but also awesome and awful. It inspired me to study weather as a career.



For those of you who are now planning to depart Australia or NZ for the tropical Islands (after the cyclone season) and would like as assisted passage: here is some good news:


Island Cruising New Zealand are organising a rally from Opua to Tonga. Sadly the registration for this is now full, but if you’d like to join the waiting list then see


And the Down Under crowd are in full swing arranging the GO EAST Cruisers rally from the Gold Coast starting around 14 May  (well after the Commonwealth Games)  to New Caledonia. For more info see


So, when will this cyclone season finish?


During the last week of March and first few days of April an MJO event travelled across the South Pacific. An MJO event is the passing of a burst of tropical convection, called a Madden Julian Oscillation or MJO. This has now gone, but there is still some “left over” tropical cyclone activity sorting itself out (see below). The interesting thing is that some computer models are forecasting the MJO oscillation to travel more quickly that normal around the world so that the next one may reach South Pacific in late April, perhaps triggering another cyclone. There does seem to be a lot of variation in the MJO models— and if the next MJO is weak or arrives after late April, then that increases the chances that this is the last week of cyclone activity for the season.


MJO phase may be seen at



After three weeks, IRIS is now fading away in the Coral sea. It still has a low-level circulation of wind, but that’s about all. Associated onshore winds on the south of IRIS have been bringing showers to the Gold Coast for the Commonwealth Games. That’s normal weather for them.


Near 17S 170E, around 100 miles ENE of Port Vila is tropical depression TD13F (Fiji Met) or 17P (Guam) continues to be a worry. Its central pressure is 998hPa, and nearest barometer at Port Vila is this evening rising rather than falling.

However, the system has persistent convection and is well placed for development and may well get named soon. It is currently doing a loop and is expected to travel southeast, skirting Fiji on Tuesday.


Fiji has been experiencing monsoonal rain during the past week with flooding in Labasa and Western division, and two dead, one missing. If this depression does visit Fiji over next few days it will be most unwelcome- and bring some wind as well as further rain— a week or so after JOSIE’s Easter visit.



A comparison of the weekly rain map

from last week with the previous week shows IRIS in the Coral Sea and the monsoonal rain over Fiji. There has also been a river of moisture extending from west of Hawaii to the USA west coast, a “pineapple express”. The “equinoctial convergence zone” between south-of-Galapagos and north-of-Marquesas is still there.


What is a Pineapple express? see



SPCZ=South Pacific Convergence zone.

The SPCZ is still intense and is expected to persist over Fiji to Samoa to Southern Cooks/French Polynesia his week. Avoid


Subtropical ridge (STR)

HIGH forming to NE of NZ on Monday is expected to travel off to the east along 35 to 40S this week.

Next HIGH is different, because it has budded off the ice shelf and the southerly winds on its leading edge are expected to shovel cold air onto New Zealand on Tuesday and Wednesday, dropping air temperatures to lowest so far this year. This HIGH should travel north across the Tasman sea on Wednesday and then east along 30S to north of NZ on Thursday and Friday.

Third HIGH this week is expected to form in south Tasman sea on Thursday and travel onto northern NZ by Saturday.


Around Tasman Sea

By mid -week the isobars are likely to be southerly from 60S to Fiji/17S. This cold outbreak is likely to be accompanied by large southerly swells from the Southern Ocean, well over 5 metres in places, but with a long period. It may trigger three smaller lows.

Another intense front is expected to cross southern NZ on Thursday and deepen into a LOW east of NZ on Friday 13th.

It isn’t really a good week from departing NZ, but those Highs in the Tasman Sea may allow a reasonable voyage between Australia and NZ.


Panama to Galapagos /Marquesas

Only light variable winds are forecast between Panama and Marquesas this week (and next week). The “equinoctial” convergence zone continues to straddle 5S from about 95W to 120W. Something to avoid.

For those trying to get from Galapagos to Marquesas, motor sail to 5S 95W and then go direct, may need to drop to 6S at times to avoid convection.


If you would like more detail for your voyage, then check to see what I offer.

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